1. Field of the Invention
This invention relates to a solid state, high frequency switching AC line voltage control apparatus, and more particularly concerns such apparatus adapted for controllably varying the alternating voltage output of the apparatus and/or stabilizing the alternating voltage output applied to a load.
2. Description of the Prior Art
Currently available AC voltage stabilizers of the electromagnetic type have large sizes and heavy weights. They introduce a considerable amount of distortion to stabilize the AC output voltage. They are extremely sensitive to variations in the frequency of the AC line input voltage, and have a very limited line regulation range, usually between 95 volts and 135 volts. They may sustain extended overload or output short circuits without material damage to themselves, but they will then dissipate large amounts of power as heat.
Another type of AC voltage stabilizer is a motor driven variac in a closed loop control circuit. This type of AC voltage stabilizer has the disadvantages of large size, heavy weight, slow response time to variations of the AC line voltage, and a low reliability factor. The slow response time is especially dangerous for the load if the AC line voltage increases sharply to an unacceptable high value. Unless an external protective device such as a reliable fuse or circuit breaker is employed, the motor driven variac cannot sustain an overload or output short circuit without being destroyed by excessive current.
An autoselected multiple tap transformer is another type of prior AC voltage stabilizer, where stabilization is accomplished in steps, rather than continuously as done by the motor driven variac. This type of AC voltage stabilizer is also large and heavy and unable to avoid damage in the event of an overload or output short circuit.
The variac employs a variable voltage transformer. This variac has a coil wound on a laminated iron core. A tap is connected to the AC line input voltage and a sliding wiper makes contact with a bare, uninsulated area of the winding. A variable output voltage appears between one end of the winding connected to the AC line input voltage and the wiper. This variac is manually controlled and like other variacs is large in size and very heavy.
Another type of motor driven variac is one where in an electric motor controls the position of the wiper moving along a winding to vary the output voltage. This variac like the others mentioned above has the disadvantages of large size and heavy weight, slow response time, and a low reliability factor, since mechanical components are employed for AC voltage control. If this variac does not have reliable external protection devices such as fuses or circuit breakers, this type of variac is subject to destruction by excessive currents due to overload or output short circuit. This type of variac is not employed in a closed feedback loop, and does not have a stabilized output.